1. Field of the Invention
The present invention relates to heaters for sheer materials, and more particularly to heaters especially suited for use in the electrophotographic process for fixing toner as transferred from a photosensitive drum onto paper.
2. Description of the Prior Art
In the so-called electrophotographic process, toner is transferred from a photosensitive drum to paper, then fused by heating with a heater and thereby fixed to the paper. The electrophorographic process has found wide use in dry copying machines, laser printers, LED printers, printing units of facsimile systems, etc.
To permit use of a compacted lightweight fixing unit in the electrophotographic process and to render the unit heatable to the operating temperature in a shortened period of time, the fixing heater, which is traditionally in the form of a tube having halogen lamp inserted therein, is replaced in some cases by a heater which comprises a strip of heating element provided on an insulating substrate. Such a hearer is disclosed, for example, in the specification of U.S. Pat. No. 5,068,517.
The disclosed heater can be produced by a simple process wherein a silver-palladium paste or the like is printed in a strip form on an insulating substrate of ceramic and then baked to form a heating resistor, is generally thin, can be heated to the toner fixing temperature instantaneously after passing a current between both ends of the resistor, and therefore has the advantage of not only providing a compact, light-weight and inexpensive fixing unit for the electrophotographic process but also necessitating little or no waiting time after the passage of current.
As shown in FIG. 8, conventional heaters of this type comprise an insulating substrate a in the form of an elongated rectangular plate, a striplike heating element b formed on the upper surface of the substrate from a resistor paste by printing and baking and having a predetermined length longitudinally of the substrate, and conductor electrodes c, c partially lapping over the respective opposite ends of the heating element b and prepared from a silver paste or like conductor paste by printing and baking.
With this structure, the heat produced by the heating element b escapes from both ends thereof to the outside through the electrodes c, c and power supply wires (not shown) connected thereto, with the result that the temperature distribution of the heating element b with respect to the lengthwise direction thereof involves a lower temperature at its opposite ends than at the intermediate portion therebetween as seen in FIG. 9. When the effective length L of the heating element b has such a reduced temperature at its opposite ends, there arises the problem that the toner becomes fixed insufficiently at opposite ends of the paper used or that fixing irregularities occur with respect to the width of the paper.
This problem will be readily overcome by sufficiently increasing the length of the heating element b relative to the effective length L of heat production and using only the uniform temperature range L' of the temperature distribution shown in FIG. 9 as an effective range of heat production.
However, the heating element so designed can not always be employed because the heating element then makes the heater itself elongated or because the fixing unit for the electrophotographic process must have a larger size to incorporate the elongated heater.
The specification of the above-mentioned U.S. Pat. No. 5,068,517 proposes another idea for correcting the temperature reduction at the heating element opposite ends due to the escape of heat from the end electrode portions, i.e., a striplike heating element b having a smaller width at its opposite ends than at the intermediate portion thereof as shown in FIG. 10. With the invention disclosed in this publiection, the opposite ends of smaller width have a greater resistance value than at the intermediate portion of large width, so that when a given current is passed through the heating element b, the rise in temperature is greater toward the opposite ends of greater resistance value. This compensates for the escape of heat from the end electrodes c, c to give a uniform temperature distribution to the heating element b in its entirety.
Nevertheless, since the striplike heating element b of the above structure locally has at its opposite ends a width smaller than the standard width of its intermediate portion, the heating element itself is inevitably weak thermally at the end portions, subjected to a marked temperature difference at the boundary between the electrode c and each element end, and liable to break owing to a thermal stress at the boundary.
Thus, it is difficult to achieve a satisfactory result in respect of the strength or life of the heating element b by the method shown in FIG. 10 of obtaining a uniform temperature distribution by decreasing the width of the heating element b at both ends thereof to give an increased resistance value.